Inertial sensors, such as accelerometers and gyroscopes, are often used to sense motion. Motion can be sensed in a linear direction in one or more axes or as rotations about one or more axes. Inertial sensors are used in a wide variety of applications.
Some micromachined accelerometers include a movable mass that has finger structures that interdigitate with fixed sensing fingers. The mass, the fixed sensing fingers, and other sensor elements (e.g., suspension structures for the mass) are typically fabricated from the same wafer and thus are nominally oriented in the same plane. Some of the elements, such as the fixed sensing fingers and fixed portions of the suspension structures, are anchored to an underlying substrate such that the fixed sensing fingers and the mass are separated from the substrate by some distance (referred to hereinafter as the “spacer gap”). Movement of the mass within the plane is typically sensed by measuring capacitance between the mass fingers and the fixed sensing fingers, with a change in capacitance reflecting an in-plane lateral acceleration. Within the plane, movement of the mass may be sensed along a single axis (i.e., an X or Y axis) or along two axes (i.e., X and Y axes).
Exemplary micromachined accelerometers of the type described above are distributed by Analog Devices, Inc. of Norwood, Mass. and are described generally in the ADXL103/ADXL203 Single/Dual Axis Accelerometer data sheet, which is hereby incorporated herein by reference in its entirety.
Some micromachined accelerometers are designed to sense motion along an axis that is normal to the plane of the substrate (i.e., the Z axis), but not along the X or Y axes. Some exemplary Z-axis accelerometers employ so-called trampoline, piston, or teeter-totter mechanisms to sense Z-axis movements.
It is often desirable to sense motion in all three axes. This is often accomplished by employing multiple single or dual axis accelerometers that are oriented along different axes. For example, each of three single-axis accelerometers can be oriented along a different one of the axes.
Accelerometers that are capable of sensing motion in all three axes are being developed. An exemplary three-axis accelerometer is described in U.S. Pat. No. 5,487,305, which is hereby incorporated herein by reference in its entirety. In this three-axis accelerometer, a separate conductor, held above the mass/finger plane by posts extending from the substrate, is used to sense Z-axis movement of the mass, while more traditional fingers are used to sense in-plane movement of the mass along the X and Y axes.